As it is known, ocular tear fluid is an organised liquid structure which coats the conjunctiva and the exposed surface of the eyeball. In normal conditions, the tear film appears to be a complex three-layered structure, comprising:
an internal layer of mucus, consisting of a mixture of glycoproteins (mucin) produced by specialised cells (i.e. the conjunctival goblet cells) which are present in the conjunctival epithelium--said layer is adsorbed on the cornea, thus forming a hydrophilic surface; PA1 a thick intermediate aqueous layer, spread over said hydrophilic surface, consisting essentially of water, electrolytes, proteins, enzymes and mucin; PA1 a thin external lipid layer, having the main function of controlling the water evaporation rate from the tear film. PA1 the necessary amount of TSP is weighed in a suitable glass vessel; PA1 90% of the available water is added, and the mixture is stirred for some hours, until complete dissolution of the product; PA1 the fixed amount of mannitol is added, while keeping stirring, and the mixture is left under stirring until complete dissolution of the product; PA1 deionised water is added up to the final weight (100%); PA1 1 N hydrochloric acid is added to reach the desired pH; PA1 the solution so obtained is sterilised in autoclave. PA1 the necessary amount of TSP is weighed in a suitable glass vessel; PA1 90% of the available water is added, and the mixture is stirred for some hours, until complete dissolution of the product; PA1 the fixed amount of mannitol is added, while keeping stirring, and the mixture is left under stirring until complete dissolution of the product; PA1 the required amount of the desired active ingredient is added while keeping stirring; PA1 deionised water is added up to the final weight (100%); PA1 the solution so obtained is sterilised in autoclave. PA1 the necessary amount of TSP is weighed in a suitable glass vessel; PA1 90% of the available water is added, and the mixture is stirred for some hours, until complete dissolution of the product; PA1 the fixed amounts of mannitol and methyl paraben sodium salt are added, while keeping stirring, and the mixture is left under stirring until complete dissolution of the product; PA1 the required amount of the desired active ingredient is added while keeping stirring; PA1 deionised water is added up to the final weight (100%); PA1 1 N sodium hydroxide is added to reach the desired pH; PA1 the solution so obtained is sterilised in autoclave.
The eyelids movement squeezes the mucus out of the conjunctival cells and introduces it into the fornices, and from there the mucus is uniformly distributed on the whole corneal surface by the blinking movements of the eyes.
The three-layers structure described above constitutes a complex physiological system, mainly directed to protect the eye surface, to maintain the hydration, the lubrication and the clearness of the corneal surface, and to cooperate in producing a correct vision. The perfect equilibrium and continuous renovation of said physiological system is a necessary condition for it to be able to carry out said functions. For the said equilibrium and renovation to be realised, a constant but not excessive water evaporation from the tear fluid must take place, so as to keep the osmolarity thereof to the physiological level of about 300 mOsm/l, and the tear film must be continuously redistributed on the corneal surface as a result of blinking.
The integrity of the internal mucin layer represents one of the essential elements of the maintenance of the tear film stability. This because mucin enhances the wettability of the corneal surface, allows the aqueous film to keep adhering to the exposed surface in a continuous and homogeneous way, thus safeguarding its stability, and increases the viscosity of the lacrimal fluid, preventing it from flowing away too rapidly from the conjunctival sac. When mucin is absent or insufficient the cornea becomes non-wettable and, as a consequence of the unbalance between electrolytes and glycoproteins present, the tear film becomes unstable and subject to breaking, with formation of dry areas.
Various diseases or abnormal conditions of the eye manifest themselves with discontinuities of the tear fluid, as a result, e.g., of an insufficient blink frequency, of the prolonged use of contact lenses, of the administration of some systemic drugs or, more frequently, of a senile hyposecretion. In this connection, the term "dry eye" syndrome is commonly used to refer to the ophthalmic condition resulting from the reduction or the instability of the tear film while, more properly, the typical alterations of the corneal surface occurring in this connection are referred to by the term "keratoconjunctivitis sicca".
In such situation a degeneration of the conjunctival cells occurs, resulting in increased desquamation, loss of the cell surface microfolds, breaking of the epithelial cells membrane and reduction of the number of mucin-producing goblet cells. This cellular degeneration, being responsible of the reduction of the density of goblet cells and of the lack of mucin, is held to be the origin of most clinical symptoms occurring in dry eye syndrome, such as dryness, irritation, photophobia and foreign body sensation.
Another phenomenon which is unanimously considered to be a sign of an irregularly structured tear fluid is the reduction of the mucus ferning. In normal conditions, mucus is characterised by crystallizing in a fern pattern when made to evaporate at room temperature from an aqueous solution. The ferning phenomenon, which is believed to result from the interaction of the electrolytes with the high molecular weight glycoproteins of mucus, is evidenced after a short time from the collection of tear mucus from the lower fornix of the conjunctiva. It has been ascertained that the various different ferning patterns (i.e., Type I, uniform ferning; Type II, good amount of ferning with ferns of reduced size and empty spaces; Type III; ferning only partially present, Type IV, ferning absent) are connected with the normal or pathological condition of the tear fluid. A dense ferning, for instance, is considered to be the expression of a perfect equilibrium between mucin and electrolytes, while the partial presence or the absence of tear ferning, which is detected in eyes affected by keratoconjunctivitis sicca, denotes a quantitative lack of tear mucus or a qualitative alteration of the glycoproteins or of their environment (i.e., pH, hydration, electrolytic equilibrium).
From a diagnostic point of view, dry eye sindrome may be detected and monitored not only by means of the evaluation of the typical symptoms thereof, but also by means of well established procedures, including, as the most common, the evaluation of lacrimal secretion (Schirmer test), the evaluation of the time needed for the tear film to break after a compete blink (break-up time. BUT), and the evaluation of the color of the corneal surface upon staining with rose bengal or fluorescein.
Keratoconjunctivitis sicca is normally treated with liquid ophthalmic preparations generally known as "artificial tears", to be instilled in drops in order to replace or supplement the natural tear production. In the simplest case said preparations have only a moistening effect, as they consist of physiological saline solutions, neutral and isotonic with the lacrimal fluid, based on sodium chloride only or on balanced mixtures of various electrolytes. An example of such a preparation, comprising at least four different ionic species (i.e. potassium, sodium, chloride and bicarbonate) in concentrations suitable to reproduce as faithfully as possible the electrolyte composition of the tear fluid, is disclosed in EP-A-0 205 279. Such preparations, as do the simpler physiological solutions, reach the objects of increasing the tear volume, moistening the ocular surface, diluting the mucus deposits and washing away any debris and foreign bodies. However, as the physiological solutions, said preparations have an extremely short duration of action (of the order of a few minutes), since the solution readily drains into the conjunctival sac. As a consequence, the instillation must be repeated every 10-15 minutes, and this brings about the patients' "non-compliance". In addition, a toxic action on the ocular tissues (conjunctiva and cornea) is exerted by the preservatives normally present in the composition.
In order to overcome the drawback mentioned above, artificial tear preparations have been introduced, which are made viscous by the addition of high molecular weight agents, such as, usually, water-soluble polymers of a synthetic, semi-synthetic or natural origin. For instance, U.S. Pat. No. 4,409,205 discloses a composition for ophthalmic use, which can serve both as an artificial tear substance and as a carrier for therapeutically active agents, wherein the viscosity enhancing agent is a non-ionic synthetic polymer, selected between polyvinyl alcohol, polyethylene glycol and mixtures thereof.
However, it has been found that, for said viscosity enhancers to confer advantageous features to a composition for use as artificial tear, is not sufficient that said viscosity enhancers generically increase the viscosity of the product, but it is also necessary that the dispersions thus formed have properties as close as possible to those of mucin dispersions. Namely, said dispersions must behave as much as possible as mucomimetic substances. This requires, fist of all, a particular rheological behaviour, i.e. non-newtonian, similar to the rheological behaviour of natural tears (see, e.g., Bothner et al., Drug Dev. Ind. Pharm., 16, 755-768, 1990). As a matter of fact, it has been shown experimentally that an artificial tear, in order to have a prolonged residence time on the corneal surface while being, at the same time, well tolerated by the patient, must not have a constant viscosity, as newtonian fluids do, but must behave as a non-newtonian pseudoplastic fluid (shear-thinning fluid), i.e. it must show a decrease of viscosity with increasing shear rate. Only such type of rheology may offer a high viscosity in the precorneal tear film at rest, so that in the absence of any stress the film adheres on the corneal surface without dropping, and, at the same time, may provide a low viscosity in the tear film during a blinking movement, when the film is subjected to a shear stress, so that the ophthalmic solution is well tolerated, and is distributed by blinking on the whole corneal surface without being massively displaced, due to friction, towards the lower eyelid rim.
The products having such pseudoplastic behaviour are characterised by a typical flow curve (i.e. the curve obtained by plotting the shear stress versus the shear rate or velocity gradient, and whose slope in each point corresponds to the viscosity value) which deviates from the straight line passing through the origin (corresponding to newtonian flow) in that it is curved with its concavity facing downwards. Such pattern corresponds to a deviation from the newtonian character in the sense of an increasing thinning with increasing shear rate.
Only a few of the macromolecular agents proposed up to now as viscosifiers for artificial tears are actually able to show a non-newtonian behaviour of the pseudoplastic type: for instance, the polyvinyl alcohol proposed by the U.S. patent document cited above gives rise, within ordinary ranges of concentration and molecular weight, to solutions which are practically newtonian.
Examples of compositions for use as artificial tears having non-newtonian rheologic behaviour are disclosed in WO-A-8404681 and in U.S. Pat. No. 5,106,615. The first document proposes the use of carboxyvinyl polymers such as Carbopol.RTM., to be included in the formulation in amounts from 0.05 to 0.25% by weight, as viscosity enhancing agents for ophthalmic solutions. The resulting solutions show, according to the said document, a non-newtonian behaviour which is currently defined as "plastic", characterised by a yield value for the shear stress, below which value no flow occurs. U.S. Pat. No. 5,106,615 discloses compositions useful both as artificial tears and as carriers for ophthalmic medicaments, which are viscosified with anionic polymers of high molecular weight (comprised between 500,000 and 4,000,000). Among the latter, the carboxyvinyl polymers mentioned above and hyaluronic acid are mentioned as preferred. Hyaluronic acid is a polysaccharide of natural origin present in many tissues and fluids, both human and animal, and largely employed in ophthalmic preparations, owing to the marked pseudoplastic behaviour of its aqueous solutions. Equally diffused as thickening agents and viscosity enhancers capable of imparting to the resulting composition the desired non-newtonian rheology are the cellulose esters, such as methylcellulose and the alcoholic derivatives thereof, e.g. hydroxypropylcellulose and hydroxypropylmethylcellulose.
As pointed out in the foregoing, in order to suitably replace and mimic the mucin component of the tear fluid, a product for use as ophthalmic solution must not only show a pseudoplastic rheological behaviour, but also it must show other properties similar to those of mucin. Among such properties there are the ability of wetting the corneal surface, which is intrinsically hydrophobic, thus increasing the uniform spreading of the tear fluid, and the ability of maintaining the integrity of the layer of tear fluid which covers the ocular surface. All that taking into account that the eye receiving the administration of an artificial tear is normally an eye with poor tear secretion, whose tear fluid contains a scarce amount of mucin. Although the products referred to above are endowed with valuable mucomimetic properties, still a good amount of product is to be administered, with a good frequency (from 6 to 12 times a day). As a consequence, the patient is still exposed to the risk of damages deriving from the preservatives which are normally present, often in combination with each other, in multiple-dose bottles.
For the above reasons there have been proposed, for the treatment of keratoconjunctivits sicca, erodible ocular inserts to be placed in the conjunctival sac. Said inserts consist, e.g., of small cylinders made of hydroxypropylcellulose which, dissolving in the conjunctival sac, continuously provide the viscosifying and lubricating mucomimetic substance. Although such inserts have the advantage of being totally free of preservatives, they can be difficult to insert, and their presence in the conjunctival sac adds to the foreign body sensation, which is always present in cases of dry eye syndrome. Furthermore, the erodible conjunctival inserts cause temporary vision disturbance, owing to the excess of polymer on the corneal surface.
In order to obtain an enhanced and prolonged lubricating action, the use of products in gel form has also been proposed (e.g., hyaluronic acid or carboxymethylcellulose gel products). However, said preparations have the drawback of blurring the vision and, therefore, they cannot be used when awake, but only while sleeping.